Wood has many advantages for structural uses. It is a renewable resource available in many parts of the world, it is easily machined to size and shape by semiskilled craftsmen using ordinary tools, it occurs naturally in large sizes, it is durable if protected from moisture or chemically treated and it is relatively light when compared with other structural materials.
In recent years, for a number of reasons, the availability of sawn wood in large sizes has diminished. This has led to the development and use of manufactured composite products such as laminated wood beams, laminated veneer lumber and other products that achieve the benefits of large size from smaller, less costly and more readily available constituent elements.
A perceived disadvantage of wood beams in structures is the combustibility of wood and therefore its potential to contribute to the spreading of fire. However:
"Timber behaves better in a fire than is often believed. It burns at the slow and fairly steady rate of 1/40 inch per minute in furnaces prescribed for American and British standard tests. It thus takes an appreciable time for a member to be sufficiently depleted to collapse, and the time, of course, increases with the size of the member."
This quote is taken from Laminated Timber, Project No. 113, European Productivity Agency of the Organization for European Economic Cooperation, March, 1953.
Other tests have shown that structures whose main supporting elements are wood beams are safer in the event of fire than equivalent buildings where the main supporting elements are steel beams. While these facts may seem to contradict common sense, they are well known to fire fighting professionals. In fires, wood beams char on their exteriors; but, this protects the interiors which continue to support the load. In the case of steel beams, the heat of a fire rapidly progresses through the beam, and the stiffness and strength diminish. This can cause an early structural collapse when the strength reduces below the value required to support the applied load.
Thus, rather than being a disadvantage, one of the primary advantages of using large wood beams in structures is the fire safety of such structures. In a building supported by large wood beams, firemen know that they have more time to rescue inhabitants and fight a fire before structural collapse than in an equivalent building using steel beams.
Laminated wood beams also have been shown to perform well in fires: "Findings from a simultaneous fire exposure of an unprotected glued laminated timber beam and a steel beam," American Institute of Timber Construction, 1961, Report of Southwest Research Institute test sponsored by the National Lumber Manufacturers Association (now American Forest and Paper Association).
Laminated beams, as well as having the advantage of using more readily available constituents, also have the statistical advantage of randomizing the locations and thereby reducing the seriousness of defects occurring naturally in wood. Thus laminated wood beams have become popular in many applications.
The large cross-sectional sizes of sawn and laminated wood beams allow them to perform well in resisting bending loads because of their moment resisting ability. The beam geometry allows the outer fibers in the plane of bending to resist bending moments with reduced stress both in tension and in compression. Between the outer fibers the shear properties of wood are sufficient to tie the outer fibers together so that the beam acts as a single unit in bending rather than as a deck of cards wherein bending loads cause a slippage of one card relative to another due to shear forces.
Several designs have been implemented in the building trades to achieve the structural advantages of large wood beams but without using as much wood. One example is the parallel chord truss used primarily in floors and flat roof systems where two long pieces of small sized lumber (the chords) are spaced parallel to one another and fastened to and braced apart with short struts (web elements). The struts are designed and positioned to resist the shear forces, and the chord elements, also known as flanges, resist the compression and tension forces resulting from bending loads applied to the truss. By properly selecting the distance of separation between the flanges, the truss can be made to be stiffer in bending and to withstand a significantly greater load than if the flanges had been joined together with no space between them. Thus, the parallel chord truss can take on the job of a heavier sawn or laminated wood beam that would use more wood. Many truss manufacturers offer parallel chord trusses as one of their structural component products.
Another structural component that achieves many of the advantages of a wood beam is the I joist. I joists are now made by a number of manufacturers. An I joist consists of two parallel flange elements, spaced apart by a web element so that the cross-sectional shape resembles the capital letter I where the flanges are the top and bottom of the I and the web is the vertical stem. The flanges are typically either solid sawn lumber or laminated veneer lumber (LVL). The web is typically plywood or oriented strand board (OSB). Joining of the flanges to the web is typically accomplished by gluing the edges of the web element into mating grooves cut into the center of one face of each of the flange elements. Both LVL and the I joist are products pioneered by the Trus Joist Corporation, now Trus Joist MacMillan, in Boise, Id.
The I joist and parallel chord floor truss concepts are similar in that they both achieve their structural values by using a web means for resisting shear forces, for supporting concentrated loads perpendicularly aligned to the beam and to space apart upper and lower flange means that resist compression and tension forces.
Another example of this concept, and the subject of the present disclosure, is the wood box beam. The wood box beam consists of two flange elements, usually, but not necessarily parallel, and two plane panel web elements also usually, but not necessarily, parallel. If the flange elements and the web elements are parallel, the box beam takes on the shape of a rectangular prism. Sometimes the bending moments in the beam are known to be less and the shear forces greater near the ends of the beam than in the middle. In those cases it may be advantageous to reduce the cross-sectional size near the ends of the beam thereby deviating from the usual prismatic shape. The web elements are rigidly fastened to the edges of the flanges typically by gluing, nailing or both. The cross section of the box beam is a closed shape which, in the usual case of a rectangular prismatic beam, is a rectangle or box; hence its name.
Because the box beam has a closed cross-sectional shape, it has much more rigidity in torsion than an I joist. Further, the two web elements and the space between them allow more options in design for resisting shear forces. For example, struts can be included between the plane panel web elements. These struts may be required in some applications to allow the box beam to withstand either or both of greater shear forces or concentrated loads.
None of the parallel chord truss, the I joist or the box beam has the fire safety advantages of a solid wood beam having equivalent structural capabilities. Consequently, the building systems in which they are used must compensate by providing fire stops or otherwise slowing the spread of flames through the structure.
Advantages of the Present Invention over the Prior Art
The box beam of the present invention retains its structural value longer when exposed to fire than either the parallel chord truss, the I joist or ordinary box beams. As an additional benefit, reinforcement can be added to increase the strength and stiffness of the beam in bending.
In some applications exposed beams are preferred for aesthetic reasons, and fire resistant box beams can serve in this application. For example in a residential basement, where the ceiling is the main floor for the structure above, the supporting joists could be wood I-joists. To give the system additional fire safety, a fire resistant plane covering, such as gypsum wall board, can be fastened to the lower surface of the I-joists. Alternatively, if fire resistant box beams are used for the supporting joists, an additional fire resistant cover may be unnecessary. Then the exposed box beams have the spaces between them as extra ceiling height contributing to the feeling of spaciousness as well as providing the appearance of a beam supported ceiling.